Field of the Invention (Technical Field)
The present invention relates to the generation of steam by direct electric heating of water containing dissolved or ionic solids. The immediate application is for generation of steam downhole in an oil bearing formation for improved production of heavy oil.
Description of Related Art
While many heavy oil formations can be stimulated to production by steam delivered from conventional surface boilers to the formation via injection tubing, there are other formations which are either too deep, or of such poor injectivity that much of the energy content of the steam is lost during delivery, and the result is poor or no stimulation. In an effort to address this problem, substantial research effort has gone into the development of steam generation downhole. For example, the U.S. Department of Energy (“DOE”) Project DEEP STEAM demonstrated operation of a downhole combustion type steam generator which utilized high pressure air, high pressure diesel and high pressure water as feedstreams to the downhole combustor and direct contact steam generator. Patents resulting from this effort can be found in U.S. Pat. Nos. 4,366,860 and 4,411,618.
A subsequent, commercial effort by many of the technologists from this initial work refined the technique to include field produced natural gas as the fuel for the downhole steam generator. Patents related to this effort can be found in U.S. Pat. Nos. 4,498,542 and 4,648,835. These efforts focused on the use of low cost fuel as an energy source, but the delivery of these feeds to a downhole steam generator, and control of these feeds from surface metering and control methods, resulted in a complicated system subject to difficulty of installation, difficulty with monitoring and control and associated high costs of materials and labor. A later reflection by one of the lead engineers for this effort can be found in “Reflections on a Downhole Steam Generator Development Program”, A. B. Donaldson, Paper 38276, presented at the Western Regional Meeting of SPE in Long Beach, Calif., Jun. 25-27, 1997, where the suggestion was made that perhaps an electric downhole steam generator could avoid many of the observed shortcomings of a combustion style downhole steam generator. The author went on to point out that if the steam generator is used primarily for pre-treatment of the well before placing on steam drive, i.e., huff-and-puff, then the cost of the energy is not an overriding consideration, whereas simplicity and ease of installation are paramount. The typical cost of electricity is around three times that of natural gas on an energy basis, because heat engines to drive generators have a thermal efficiency of only around 35%-40%. However, during off-peak periods, electric energy may be much cheaper because power generating companies desire to keep generating assets base-loaded. The simplicity of an electric steam generator is such that it can easily be shut down and restarted to enjoy discounted off-peak electric rates. And with this possible cyclic operation, injection of other fluids, e.g., water or air, may produce enhanced oil production owing to a symbiotic effect; these suggested alternating cycles are not novel, except in association with a downhole electric steam generator. For example, water injection can moderate formation temperatures in the circumstance that the equilibrium steam temperature in association with the required injection pressure, exceeds the temperature which is needed for adequate viscosity reduction. Water injection will assist in advancing the heated zone farther into the oil formation while the generator is on standby. Air injection can initiate cyclic combustion which can be controlled by subsequent daily steam injection, thus minimizing channeling, which is a known problem with in-situ combustion projects.
The utilization of direct electric heating of water for downhole steam generation was disclosed in U.S. Pat. No. 4,783,585 based on a two electrode design, and the general strategy has been otherwise mentioned, including in U.S. Pat. No. 5,623,576. A later Master's Thesis (“Direct Electric Heating of Electrolyte Solutions”, by Ahmed Mijbel Kanaan, M S Thesis, New Mexico State University, 2010) demonstrated a continuous flow steam generator utilizing 240 vac power, with two electrode plus neutral design and using a typical domestic water supply. This research indicated that water hardness can beneficially be removed by a conventional water softener before passing into a steam generator.